Particles are frequently modified from one form to another in a continuous multi-stage process. Often, this involves removing a material from the particles, but may also involve chemically altering the particles or a constituent in the particles.
One or more of these stages in such a process may use a processing fluid (liquid or gaseous) to modify the particles. For example, a process for decaffeinating coffee beans requires solvent stages and solvent removal stages to produce the final decaffeinated beans. The solvent, even the same solvent, may be liquid or gaseous at different points in a typical process.
Often, the processing steps upstream of a stage using a processing gas (i.e., gaseous material) will not operate properly if the gas in a downstream stage propagates to upstream stages. For example, the downstream gas may inactivate the upstream stage or even form a toxic or dangerous mixture with either the particles in the form they are at that stage or with a processing fluid in the upstream stage. If an upstream stage uses high heat, a combustible gas used in a downstream stage must not be allowed to backflow to the heated upstream stage.
Accordingly, it is sometimes necessary to prevent backflow from a higher-pressure downstream stage to the immediate upstream stage. One easy way of course is to simply control the gas pressure of each stage so that the gas pressure in each downstream stage is lower than the adjacent upstream stage. However, where a number of stages are involved, this may not be possible. Even if only two stages are involved, the chemistry of the stages' processes may not allow the downstream stage to operate at a lower pressure than the adjacent upstream stage.
A number of mechanical devices are available to oppose such backflow of gasses. However, for the most part, these devices do not totally prevent backflow of gasses. For example, a rotary gate allows some leakage through the gate seals.
If a near perfect seal is required between two stages of a continuous particle process, no system presently available is completely satisfactory.
It is known that a column of particles held in a duct or tube is permeable by a fluid. For example, a liquid poured into the top of a duct holding a column of particles will percolate to the bottom of the duct. The rate of percolation depends in part on characteristics of the particles, but also on how tightly the particles are packed.
Similarly, if a gas pressure difference is created between ends of a duct completely filled with particles, gas will percolate through the column of particles from the higher pressure end to the lower pressure end. The mass rate of gas percolation and the velocity at which the gas percolates depends on how densely packed the particles are, the shape and size of the particles, and the pressure difference.